The invention relates to an apparatus for pressure treating a moving web by pressing a length of the web between a surface and a moving chain.
One example of the pressure treatment of a moving web is found in the nip press section of a paper making machine or a pulp machine. The continuous fiberous web is normally pressed between at least two closely spaced-apart press rolls. The pressure applied to the moving web by the rolls extracts water from the web. It has been recognized that a greater percentage of water can be extracted from the web by increasing the time during which the web is under pressure in the nip press section. There have been attempts to accomplish this by increasing the length of moving web which is under pressure and thereby the time available for drainage. In most cases, it has not been possible to reach more than 40% to 45% dryness. However, it is known that under static conditions paper webs can be pressed to over 70% dryness.
Attempts have been made to increase the nip width and thus the time under pressure by the use of softer rubber coverings on press rolls or otherwise deformable rolls. An example of this is found in U.S. Pat. No. 3,804,707 to Mohr et al. Such attempts have met with limited success due to the creation of sheer forces acting on the web on the ingoing and outgoing sides of nips.
Another approach is to replace one of the press rolls with a pressing device having a concave shape which is complementary to the cylindrical surface of the roll. Moving endless belts have been used to provide this complementary concave shape as seen for example in U.S. Pat. No. 3,808,096 to Busker which shows a figure eight-shaped cyliner press. Some means has normally been provided to press the belt against the press roll to give sufficient pressure on the web for the dewatering operation. In the Busker patent, for example, means is provided for delivering fluid under pressure to fill the belt loop. Fluid pressure chambers are applied in U.S. Pat. Nos. 4,173,249 to Holkko, 3,808,092 to Busker and 3,293,121 to Martin. The use of stationary backing members for applying pressure to such a belt is disclosed in U.S. Pat. No. 4,201,624 to Mohr. In U.S. Pat. No. 3,853,698 to Mohr, a pressure chamber is used.
Attempts to use belts for extended nip presses have not always been completely successful. U.S. Pat. No. 4,238,287 to Gill and U.S. Pat. No. 4,229,253 to Cronin disclose reinforced belts for extended nip presses.
U.S. Pat. No. 2,890,748 to Heinrich shows the use of a link chain belt for drainage in nip presses. However, Heinrich does not disclose the possibility of using chains to support a belt in an extended nip press or to provide the concave-shaped press member which has a shape complementary to that of the roll.
Other United States patents in the field include U.S. Pat. Nos. 3,325,351 to Orton, 3,528,883 to Lundin and 3,566,781 to Kunze.
Problems have been encountered with many prior art extended nip presses and so they have not been widely adopted. For example, there is inherent friction between stationary supports and the belt which leads to problems associated with resistance to the movement of the belt such as the heating thereof. Lubrication between the support shoe and the belt may be provided to ameliorate the situation. Reliability may be restricted in some systems using fluid support for the belt because of complexity and sealing problems. Accordingly, there is a need to provide improved or alternative support means for the web in an extended nip press.